The present invention relates to a device of changing a running path of a lower thread for desired zigzag stitches.
In zigzag stitchings of the prior art, a positional relation between stitching and a thread catching device is as shown in FIG. 6, as seen from a top of a stitching bed, and a lower thread exit 20 is offset to the left side with respect to a needle amplitude (B-C).
Therefore, consumption amounts of required lower threads in zigzag stitching are different in the needle lateral amplitudes from B to C and from C to B. Assuming that the lower thread exit 20 is A and a feed amount is 0, the former consumption amount is BC+AC-BA, while the latter consumption amount is BC+BA-AC, and since the consumption amount by the former is larger, the stitches are formed as shown in FIG. 7 in that the lower thread is biased to the left side.
To avoid biasing of the lower thread, if the lower thread exit 20 is provided at the center as shown in FIG. 8, the consumption amount of the lower thread is made equal in the both needle amplitudes, that is the needle is moved from B to C and from C to B, and stitches as shown in FIG. 9 may be formed.
However, sometimes when a straight stitching is carried out, abnormal stitches can be formed as shown in FIG. 10.
Such abnormal stitches can be avoided by providing the lower thread exit hole 20 which is offset from the center as shown in FIG. 6 (see FIG. 11).
Thus, different exits for passing the lower thread are required to be suited to the zigzag stitching and also the straight stitching.